What is (+) in PL/SQL means ?

Hi all,
I have a stored procedure written by someone else. Some select statements in this SP the where clause contains (+), e.g.

    select s1.service_id, s2.service_id

      from services s1,
           services s2,
           service_profile sp
     where s1.service_plan = parBillCode
       and s1.service_id   = sp.service_id (+)
       and sp.category (+) = 'PLAN_AAA'
       and sp.parameter    = s2.service_plan (+);

I can't search any reference because search engine seems to ignore (+) characters...
(I use google search, type in ("(+)" PL/SQL) in the textbox, but the result only contains "PL/SQL"....) Received on Sun Feb 17 2002 - 15:50:27 CST